John Ashurst - Conservation of Building and Decorative Stone

Conservation of Building and Decorative Stone

Butterworth-Heinemann Series in Conservation and Museology

Series Editors: Arts and Archaeology

Andrew Oddy British Museum, London

Architecture

Derek Linstrum Formerly Institute of Advanced Architectural Studies, University of York

US Executive Editor: Norbert S Baer New York University, Conservation Center of the Institute of Fine Arts

Consultants: Sir Bernard Feilden

David Bomford National Gallery, London

C V Horie Manchester Museum, University of Manchester

Colin Pearson Canberra College of Advanced Education

Sarah Staniforth National Trust, London

Published titles: Artists' Pigments c. 1600-1835, 2nd Edition (Harley) Conservation and Exhibitions (Stolow) Conservation and Restoration of works of Art and Antiquities (Kiihn) Conservation of Building and Decorative Stone (kshurst, Dimes) Conservation of Glass (Newton, Davison) Conservation of Historic Buildings (Feilden) Conservation of Libran and Archive Materials and the Graphic Arts (Petherbridge) Conservation of Manuscripts and Paintings of South-east Asia (Agrawal) Conservation of Marine Archaeological Objects (Pearson) Consenlation of Wall Paintings (Mora, Mora, Philippot) The Museum Environment, 2nd Edition (Thomson) The Organic Chemistn of Museum Objects (Mills, White) The Textile Conservator's Manual (Landi)

Related titles: Manual of Curatorship Materials for Conservation Museum Documentation Systems

Conservation of Building and Decorative Stone

Editors

John Ashurst D.Arch, RIBA, EASA (Hon) Formerly Principal Architect, Research and Technical Advisory Service, Historic Monuments Commission for England. Now private consultant in the field of historic building repair.

Francis G Dimes MSc BSc FGS The late Francis Dimes was a Consultant Geologist in private practice, and formerly Curator of Building Stones at the Geological Museum in South Kensington, London.

E I N E M A N N s OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

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Series Editors' Preface

The conservation of artefacts and buildings has a long history, but the positive emergence of conservation as a profession can be said to date from the foundation of the International Institute for the Conservation of Museum Objects (IIC) in 1950 (the last two words of the title being later changed to Historic and Artistic Works) and the appearance soon after in 1952 of its journal Studies in Conservation. The role of the conservator as distinct from those of the restorer and the scientist had been emerging during the 1930s with a focal point in the Fogg Art Museum, Harvard University, which published the precursor to Studies in Conservation, Technical Studies in the Field of the Fine Arts (1932-42).

UNESCO, through its Cultural Heritage Division and its publications, had always taken a positive role in conservation and the foundation, under its auspices, of the International Centre for the Study of the Preservation and the Restoration of Cultural Property (ICCROM), in Rome, was a further advance. The Centre was established in 1959 with the aims of advising internationally on conservation problems, co-ordinating conservation activities and establishing standards and training courses.

A significant confirmation of professional progress was the transformation at New York in 1966 of the two committees of the International Council of Museums (ICOM), one curatorial on the Care of Paintings (founded in 1949) and the other mainly scientific (founded in the mid-1950s) into the ICOM Committee for Conservation.

Following the Second International Congress of Architects in Venice in 1964 when the Venice Charter was promulgated, the International Council of Monuments and Sites (ICOMOS) was set up in 1965 to deal with archaeological, architectural and town planning questions, to schedule monuments and sites and to monitor relevant legislation.

From the early 1960s onwards, international congresses (and the literature emerging from them) held by IIC, ICOM, ICOMOS and ICCROM not only advanced the subject in its various technical specializations but also emphasized the cohesion of conservators and their subject as an interdisciplinary profession.

The use of the term Conservation in the title of this series refers to the whole subject of the care and treatment of valuable artefacts both movable and immovable, but within the discipline conservation has a meaning which is distinct from that of restoration. Conservation used in this specialized sense has two aspects: firstly, the control of the environment to minimize the decay of artefacts and materials; and, secondly, their treatment to arrest decay and to stabilize them where possible against further deterioration. Restoration is the continuation of the latter process, when conservation treatment is thought to be insufficient, to the extent of reinstating an object, without falsification, to a condition in which it can be exhibited.

In the field of conservation conflicts of values on aesthetic, historical, or technical grounds are often inevitable. Rival attitudes and methods inevitably arise in a subject which is still developing and at the core of these differences there is often a deficiency of technical knowledge. That is one of the principal raisons d'gtre of this series. In most of these matters ethical principles are the subject of much discussion, and generalizations cannot easily cover (say) buildings, furniture, easel paintings and waterlogged wooden objects.

A rigid, universally agreed principle is that all treatment should be adequately documented. There is also general agreement that structural and decorative falsification should be avoided. In addition there are three other principles which, unless there are overriding objections, it is generally agreed should be followed.

The first is the principle of the reversibility of processes, which states that a treatment should normally be such that the artefact can, if desired, be returned to its pre-treatment condition even after a long lapse of time. This principle is impossible to apply in some cases, for example where the survival of an artefact may depend upon an irreversible process. The second, intrinsic to the whole subject, is that as far as possible decayed parts of an artefact should be conserved and not replaced. The third is that the consequences of the ageing of the original materials (for example 'patina') should not normally be disguised or removed. This includes a secondary proviso that later accretions should not be retained under the false guise of natural patina.

The authors of the volumes in this series give their views on these matters, where relevant, with reference to the types of material within their scope. They take into account the differences in approach to artefacts of essentially artistic significance and to those in which the interest is primarily historical or archaeological.

The volumes are unifled by a systematic and balanced presentation of theoretical and practical material with, where necessary, an objective comparison of different methods and approaches. A balance has also been maintained between the fine (and decorative) arts, archaeology and architecture in those cases where the respective branches of the subject have common ground, for example in the treatment of stone and glass and in the control of the museum environment. Since the publication of the first volume it has been decided to include within the series related monographs and technical studies. To reflect this enlargement of its scope the series has been renamed the Butterworth-Heinemann Series in Conservation and Museology.

Though necessarily different in details of organization and treatment (to fit the particular requirements of the subject) each volume has the same general standard which is that of such training courses as those of the University of London Institute of Archaeology, the Victoria and Albert Museum, the Conservation Center, New York University, the Institute of Advanced Architectural Studies, York, and ICCROM.

The authors have been chosen from among the acknowledged experts in each field, but as a result of the wide areas of knowledge and technique covered even by the specialized volumes in this series, in many instances multi-authorship has been necessary.

With the existence of IIC, ICOM, ICOMOS and ICCROM, the principles and practice of conservation have become as internationalized as the problems. The collaboration of Consultant Editors will help to ensure that the practices discussed in this series will be applicable throughout the world.

Preface

In presenting this book, John Ashurst and I have many people to thank, particularly Ian Bristow for his admirable introduction to the subject of conservation on which successive contributors have built; and David Honeyborne, whose name has long been associated with pioneering work in the field of stone weathering. Many others are owed much, and it is a debt which cannot adequately be repaid.

For my part I must thank John Ashurst, first and foremost for the constant aid and encouragement he has given. Without him the shoe leather would not have hit the pavement carrying the wearer to yet another building to add to the record. Chapters 2 to 6 owe much to Murray Mitchell, one of Britain's foremost geological editors, who spent many hours reading the script, correcting it, improving it and removing the double-negatives. The script was better for it. Especial thanks are due also to a friend and former colleague, Gilbert Green, for his great pati- ence in straightening out my understanding of Bath Stone. The section on that stone owes much to him.

My erstwhile colleagues at the Geological Museum have helped, probably more than they realize, in answering the questions asked of them. Ron Roberts, Alan Timms, Peter Clough, Alan Jobbins and Robin Sanderson all had the kindness not to plead other engagements when they saw the question coming.

Inevitably Chapters 2 to 6 of Volume 1 are to some degree a compilation of existing, scattered knowledge. Many of the examples given are quoted from other books. They are listed in the References. There are examples, however, which may be said to be in the public domain. Should any of my friends have passed an example to me and I have not acknowledged it, I offer them my apologies.

It has not been possible to check every example given of the use of stone. I should be grateful, therefore, if any reader would let me know of any which are incorrect and, indeed, I should be pleased to learn of new outstanding uses. None of this might

Donovan Purcell

have happened but for one person to whom both John Ashurst and I must express the deepest debt of gratitude. Sadly he is no longer with us. Donovan Purcell, a former Surveyor to the Fabric of Ely Cathedral, a friend in all senses of the word, gently taught me what it was that an architect wanted to

know from a geologist. To both John and me he demonstrated what limitless enthusiasm for a material could achieve. As founder chairman of the Standing Joint Committee on Natural Stones he opened a dialogue again between the masonry trade, the quarrying industry, the training establishments and architects and surveyors, contributing in no small way to a new unity of interest and purpose which has been foundational to the revival of the stone industry of the United Kingdom. To a great extent this book is part of his memorial.

John Ashurst and I have learned much from the band of dedicated people who have made their knowledge available and who have contributed largely to Volume 2. John particularly remembers

with great pleasure the many discussions with these contributors on the philosophy of repair, consolidation and conservation, That this book is in two volumes is simply a matter of convenience. There is no division between an understanding of the nature of stone and the study of its repair and conservation in buildings.

Finally I must thank Janet, who came into the life of this book - too late! - and Margaret, who has lived with this book - too long!

Francis G. Dimes Kingston Vale,

September 1990

Preface to paperback edition

Frank Dimes died on 8 October 1995. He and I both hoped to see Conservation of Building and Decora- tive Stone in a paperback edition and he would have been delighted to see that hope fulfilled. Had he been here I know he would have wanted to enlarge and enrich his sections of the book; he was always collecting new material with unabated enthusiasm and we discussed many 'improvements' over pints of Young's Special. These additions were not, as it happens, to be possible, but his work stands still as a testament to Frank's enormous experience and lucid- ity. Through his writing, some of which has been published posthumously (The Building Stone Herit-

age of Leeds with Murray Mitchell) and some of which may still be published, he is still able to educate and inspire and to entertain as he did throughout his professional life. He is still sadly missed by his wife Margaret and their daughters, by all his many friends and associates, and not least by me, remembering him as my greatest mentor. I am proud to have known and worked with him.

John Ashurst Epsom

November 1997

part 1

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Contents

1 An introduction to the restoration, conservation and repair of stone 1 Ian Bristow

2 The nature of building and decorative stones 19 Francis G. Dimes

3 Igneous rocks 37 Francis G. Dimes

4 Sedimentary rocks 61 Francis G. Dimes

5 Metamorphic rocks 135 Francis G. Dimes

6 Determination of a sample 150 Francis G. Dimes

7 Weathering and decay of masonry 153 David B. Honeyborne

Illustrations of weathering and decay phenomena 179

Index 185

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An introduction to the restoration, conservation and repair of stone

Ian Bristow

Introduction

Whoever expects to find a stone that will stand from century to century, deriding alike the frigid rains and scorching solar rays, without need of reparation, will indeed search for 'the philosopher's stone'.'

As C.H. Smith, who delivered these lines at a lecture to the Royal Institute of British Architects in 1840 realized, stone, despite its image as the eternal material, has a limited life. Its decay may be caused by a number of factors, including polluted acidic atmospheres, which lead to surface erosion, flaking, and exfoliation. Eventually this may impair the aesthetic appearance of a building or affect its structural stability, and proper remedial measures will be needed.

The traditional method of repair has been to cut out and renew all weathered or otherwise defective stones, but in the case of historic buildings this process is, in effect, destructive both of the ancient fabric, with its archaeological interest and, no less importantly, of the character of age such a structure will inevitably possess. The loss of either will concern equally those who are interested in the history of a building and those to whom the qualities of its age are important. It is therefore necessary to adopt a special attitude to the repair of historic stonework, soundly based on a carefully considered philosophy. The approach adopted in any given case will vary, but should spring from consideration of a number of points, most importantly:

1. the age and character of the building; 2. the structural function of the individual stone in

question, and the nature and cause of its defect; 3. a careful assessment of its rate of decay, taken in

the context of the building as a whole.

The last of these, the much overlooked dimension of time, is in many ways the most difficult. It needs experience and an outlook which is foreign to many architects and building contractors, especially today; but it was much neglected too in the nineteenth century, and many churches received over-drastic restoration as a result.

The first essential in approaching an historic building in need of repair is to determine the cause of decay, and, where possible, remove it or minimize its effect. If surface disruption is caused by acidic air pollution, there is little the individual can do, although governmental action in a national or international context could be of the very greatest importance. Poor selection or incorrect bedding of stones is also without simple remedy, but it is sometimes possible to improve poor detailing without alteration to the appearance of a building, as, for example, by the provision of a drip on the underside of a projecting window sill. Where a material change in appearance would result, however, it is often necessary to accept the defect and any consequent tendency to decay in order to preserve the integrity of the historic fabric.

The effects of overloading can, on the other hand, often be remedied inconspicuously. For example, the detached marble shafts employed in thirteenth- century churches and nineteenth-century buildings of the same style often become overcompressed through settlement of the more frequently jointed adjoining masonry, and the stress may be simply relieved by sawing out their joints and repointing. To remedy the spalling resulting from concave beds, or the decay caused by juxtaposition of incompatible stones is, however, more problematical; but action can be taken to control plant growth in masonry, rusting and contingent expansion of iron cramps or window ferramenta, damage caused by mason bees, leaking gutters and rainwater goods, rising damp, and unsuitable uses and human activities.

2 An introduction to the restoration, conservation and repair of stone

In practice, one of the most important responsibili- ties of anyone faced with the care of an historic building is to anticipate trouble before it happens. Not only must maintenance, especially the cleaning out of gutters and downpipes, be carried out thoroughly and regularly, but potential trouble spots should be eliminated. If a bend can be removed from a rainwater pipe it will lessen the chances of a blockage; and where there is important internal masonry, such as a carved freestone or marble wall monument, any exterior downpipe should be resited as far away from it as possible. Care should also be taken to ensure that any repointing is carried out in a suitable mix, and that impervious paints and plasters, which will inhibit the free evaporation of moisture, are not used to the detriment of the masonry. Heating pipes should be sited well away from historic carving or wall bases in order to avoid rapid decay through increased evaporation of moisture within the fabric.

The birth of a conservative approach to repair

The need for careful maintenance was recognized by the fifteenth-century Florentine architect Leone Bat- tista Alberti, who exclaimed that he was often filled with the highest indignation when he saw buildings going to ruin owing to the carelessness of their ~ w n e r s . ~ From the mid-sixteenth century, however, the growing English fashion for Italianate architecture led to a contempt for the Gothic style, and its consequent neglect. The seventeenth-century diarist John Evelyn, for example, spoke disparagingly of Henry VII's chapel at Westminster, as being composed of 'lame Statues, Lace and other Cut-work and Crinkle rankle';^ and at the same period many mediaeval churches, already despoiled by puritanical fanaticism, fell into poor repair. The Civil War too brought in its wake a toll of destruction to many castles, and a large number were slighted to prevent their continued use for military purposes.

The resulting ruins, together with those of classical Italy, soon became a poignant reminder of the past; and their fascination to the eighteenth-century m'ind is splendidly revealed in the following lines from David Mallet's poem The Excursion of 1726:

Behind me rises huge an awful Pile, Sole on this blasted Heath, a Place of Tombs, Waste, desolate, where Ruin dreary dwells, Brooding o'er sightless Sculls, and crumbling

Bones. Ghastful He sits, and eyes with stedfast Glare The Column grey with Moss, the falling Bust,

The Time-shook Arch, the monumental Stone, Impair'd, effac'd, and hastening into ~ u s t . *

The particular qualities of ruins eventually became incorporated formally into aesthetic theory. In the 1750s, the philosopher Edmund Burke had postula- ted two characters, the Sublime and the Beautiful, the latter expressed by smooth outline and flowing lines, the former by jagged outline and grandeur of scale, a quality with which ruins would no doubt have been identified.5 By the 1790s, however, this simple duality had come to seem unsatisfactory, and Sir Uvedale Price added a third character, the Picturesque, describing the way a beautiful building with its smooth surface and even colouring was converted by time into a picturesque ruin:

First, by means of weather stains, partial incrusta- tions, mosses, &c. it at the same time takes off from the uniformity of its surface, and of its colour; that is, gives it a degree of roughness, and variety of tint. Next, the various accidents of weather loosen the stones themselves; they tum- ble in irregular masses upon what was perhaps smooth turf or pavement, or nicely trimmed walks and shrubberies; now mixed and over- grown with wild plants and creepers, that crawl over, and shoot among the fallen ruins. Sedums, wall-flowers, and other vegetables that bear drought, find nourishment in the decayed cement from which the stones have been detached: Birds convey their food into the chinks, and yew, elder, and other berried plants project from the sides; while the ivy mantles over other parts, and crowns the top."

The late eighteenth and early nineteenth century greatly enjoyed the qualities offered by decay of this nature, and the preoccupation with its pleasing character is well expressed in engravings of the period, such as that of St. Giles's Church, Little Malvern, Worcestershire (Figure 1 .1 ) . In it may be seen the evident pleasure of the artists in the ivy-clad ruins at the east end and on the southern side of the chancel of this fifteenth-century building, besides the truncated tower with its pyramidal roof which replaced the earlier parapet.

In parallel with this enthusiasm for the aesthetic qualities of ruins, an academic interest in the study of the remains of mediaeval architecture developed. The eighteenth-century classical architect Sir Wil- liam Chambers, for example, made a plea in his Treatise on the Decorative Part of Civil Architecture ( 179 1 ) for 'a correct elegant publication of our own cathedrals, and other buildings called Gothick, before they totally fall to ruin',' which was answered rapidly by a whole series of publications. Under the auspices of the Society of Antiquaries, John Carter

The birth of a conservative approach to repair 3

Figure 1.1 Pleasing decay. A view of St Giles's Church, Little Malvern, Worcestershire. (Engraving by J.1e Keux from a drawing by J.P. Neal in their Views of the most interesting Collegiate and Parochial Churches in Great Britain ( 1824-1825), volume 2 )

produced his superb folios of measured drawings of cathedrals, including those of Durham, Exeter, Gloucester, and York, and the abbeys of Bath and St. Alban's; whilst in 181 1 John Milner published his Treatise on the Ecclesiastical Architecture of England, a work followed in 1817 by Thomas Rickman's Attempt to discriminate the Styles of Architecture in England, which formed the foundation of nineteenth-century scholarship. Perhaps the most prolific publisher of the period was John Britton, whose Architectural Antiquities, which contained splendid engravings of ancient buildings, appeared in five volumes between 1807 and 1826, and whose series of Cathedral Antiquities, which dealt with fourteen English cathedrals, was produced between 1814 and 1835. The latter volumes were distinguished by containing, besides a selection of general views and details, carefully executed measured drawings of the buildings. This was also a feature of the vitally important volumes of Specimens and Examples of Gothic Architecture produced by Augustus Pugin in 1819-1822 and 1828-1838 respectively. Together with others, these formed primary source books for the revival of mediaeval architectural styles, a subject outside the scope of the present chapter, but brilliantly charted by Charles Eastlake in his Histoly of the Gothic Revival (1872). It had, though, an important parallel which

is of great moment in the present context, the revival by the Tractarians of the ancient dignity of Christian worship.

In 1827, the author of Notes on the Cam- bridgeshire Churches wrote:

The dilapidation of churches is a delicate subject to speak of ... but when the archdeacons aban- doned their duty . . . peculation, ruin, and desola- tion stalked abroad.. . and corruption and decay withered all around. .. The established places of worship have become unfit and unsafe for Christ- ians to meet in; the churches are cold, comfort- less, unhealthy; the haunts of colds, catarrhs, and rheumatism; the receptacle frequently of filth, and the abode of toads and reptiles. Congregations.. . are deterred from entering.. . by the dread of the fevers and consumption that they know lurk within.. . [and] are driven into dissenting places of worship.'

A few years later, Augustus Pugin's son, Augustus Welby Northmore Pugin, complained that the font at Selby Abbey, Yorkshire, was disused. The transept chapels were filled with rubbish, one even being used as a coal hole, and the eastern aisle windows were disfigured by having two large stove pipes carried through them.' The engraving of the interior


mediaeval ecclesiastical structures, but, most importantly, to undertake their restoration. They started their programme for the latter in a small way with the font at Coton, a village church very close to Cambridge," and in 1841 reported their repair of the font at St. Peter's. This had been broken into pieces, some of which had been lost;'' the restored bowl with its sculpture pieced-in by the Society can still be seen in the church today. In June 1840, they paid to have the rough-cast removed from the tower of St. Bene't's, Cambridge, in order to expose 'the interest of its Anglo-Saxon c~nstruction','~ and the following year embarked on their largest project, the 'thorough restoration' of the Church of the Holy ~ e ~ u l c h r e , ' ~ which, on account of the impact it was to have elsewhere, is worth considering in a little detail. The opportunity for the Society's involvement was provided by the collapse of part of the vault of the circular aisle, caused by settlement of the perimeter wall which had been undermined by grave-digging. This had occasioned movement in the round tower, and the remedial measures instigated in 1841, under the direction of the architect Anthony

Figure 1.2 The neglected state of St Peter's, Cambridge, in the early nineteenth century (provenance unknown)

of St. Peter's Church, Cambridge, illustrated in Figure 1.2, shows the appearance of just such a neglected church; and it was not long before a concerted effort was made to remove the offending bric-a-brac from them, to repair their roofs and restore their damaged masonry in order to permit seemly and proper worship within. A large number of churches had, of course, been refitted in the eighteenth century, most generally by the installation of box pews, a reredos, pulpit, and galleries; and certain major structures, such as Milton Abbey, Winchester and Salisbury Cathedrals and Henry VII's Chapel, had been restored by architects such as James Wyatt, whose thoroughgoing approach had been the subject of controversy at the time.'' To those who wished to revive the ancient dignities of worship, however, such crass alterations were anathema, and the resulting movement to put matters to rights led in 1839 to foundation of the Cambridge Camden Society. This event was of profound significance and was soon to have a devastating effect on ancient structures throughout the length and breadth of Britain. Figure 1.3 Church of the Holy Sepulchre, Cambridge,

The Society was formed by a group of undergradu- before restoration (from John Britton, The Architectural ates, notable amongst whom was J.M. Neale, who Antiquities of Great Britain (1807.1826) volume 3 dedicated themselves not only to the study of (1812), plate facing page 90)


Salvin, involved not only rebuilding the wall and vault, but also the removal of the upper storey of the tower which had been added to the Romanesque structure in the fifteenth century (Figure 1.3). In addition, 'Plain single Norman lights' were 'substi- tuted for the unsightly Perpendicular insertions which disfigured, as well as weakened, the walls of the circular Aisle'. The liaison in this statement between a return to a structurally perfect condition and restoration to an earlier physical state is significant; and in parallel with this, the box pews and other later furnishings were removed from the interior.

Such an approach typified a large number of later restorations at other churches inspired by the activities of the Cambridge Camden Society, the nineteenth-century debate surrounding which has been ably charted by Stephan Tschudi Madsen in his recent book Restoration and Anti-Restoration (Oslo, 1976), to which the reader is referred for greater detail of this important issue. From the point of view of the present chapter, however, the most important facet of the Holy Sepulchre restoration was the smoothing and redressing of the remaining ancient stonework both within and without, a matter the Society reported with enthusiasm.15 The transformation effected by the works may be seen in the post restoration photograph (Figure 1.4), the final product, bereft of patina and 'scraped' clean, presenting a tidy and perfect face to the world, thus testifying to its new found health in the care of what was seen by its authors as a revitalized Christian witness.

Although the Cambridge Camden Society was disbanded in 1846, it was refounded the following year in London as the Ecclesiological Society and continued its activities, which included publication of the periodical, the Ecclesiologist (1841-1868). Through this, the principles of restoration advocated by the Society swept the country, and church after church was subjected to thoroughgoing works of the kind seen at the Holy Sepulchre. The enthusiasm of the clergy, which stands in marked contrast to their general apathy towards historic buildings today, ranged unchecked over ancient fabric; and gradually the picturesque face of the English parish church became transformed into a scraped and tidy bland- ness, scarred too with the harshness of new stone which was often cut with a soulless precision unknown in the Middle Ages. Restoration thus came inevitably into conflict with artistic sensibilities, and, furthermore, with archaeological interests as genuine mediaeval work disappeared in favour of somebody's more or less scholarly notion of what seemed correct.

The most notable reaction to this process was that of John Ruskin, who was horrified at the destruction of ancient fabric which was taking place. In 1849 he expressed his concern with considerable force in the 'Lamp of Memory', which formed Chapter VI of the Seven Lamps of Architecture. He characterized restoration as 'a Lie from beginning to end' and wrote:

You may make a model of a building as you may of a corpse, and your model may have the shell

Figure 1.4 A nineteenth-century photograph showing the Church of the Holy Sepulchre, Cambridge, after restoration


of the old walls within it as your cast might have the skeleton.. . but the old building is destroyed.

To lose the original surface, albeit weathered, was tragic; and its replacement totally unsatisfactory. How could one, he demanded, copy a surface that had been worn half an inch down, since the whole finish was in the half inch that had gone. The old, he insisted, still had some life, some mysterious sugges- tion of what it had been, and of what it had lost. All in all, he remarked:

Neither by the public, nor by those who have the care of public monuments, is the true meaning of the word restoration understood. It means the most total destruction which a building can suffer.'"

In both the Seven Lamps and the Stones of Venice, which succeeded it a few years later in 1851-1853, Ruskin showed himself to be not only an extremely sensitive observer and draughtsman of the patina of decay, but also an astute analyst of mediaeval fabric. The irregularities of setting out which he discovered at Pisa and elsewhere .were just the sort of thing liable to be missed and made uniform by a restorer," and the almost unique way in which he combined picturesque artist and archaeological scholar was quite remarkable.

The force of his arguments come to be accepted only gradually, to the great regret of many architectural historians today, but by 1861 the architect William Butterfield was writing in connection with his restoration of the tower of the Chapel at Winchester College:

I should carefully save and reuse every old moulding and surface stone which is at all likely to last, even though it may be in some respects in an imperfect state;''

whilst about five years later George Edmund Street, faced at Monkland, Herefordshire, with tufa 'so rough, and ... so rude, that most men would have proposed to build an entirely new church', rebuilt the nave 'with every wrought stone put back in its old place'.19 One of the most revealing of these later restorations was the anastylosis, carried out by Sir Gilbert Scott in about 1875, of the fragments of the shrine of St. Alban which had been discovered in 1873. This was later praised even by such a critic as E.S. Prior, who remarked that we are

fortunate in the taste and good sense with which the remains have been put together and treated, as such monuments should be, with the sole intention of the preservation of the beauties they have left.*"

Scott himself wrote that the Shrine was, 'by the ingenuity of the foreman and the clerk of the works,

Figure 1.5 St Alban's Abbey, Hertfordshire, the shrine of St Alban after reconstruction in the 1870s. (From Architecture, volume 2 (1897), p. 77)

set up again, exactly in its old place, stone for stone, and fragment for fragment: the most marvellous restitution that ever was made'.2' The illustration (Figure 1.5) shows the result, with no attempt to replace missing features, nor, most importantly, to renew damaged or defective stones. Of it, Gilbert Scott's son, George Gilbert Scott wrote:

In that one structure, as it now stands, is summed up the history of english church architecture as a living fact, and of the death which finally overtook it. The one is seen in the exquisite finish and beauty of the monument thus recovered from its ruins; the other in the marks, which it bears upon it, of the crowbar-blows which shattered it into splinters, starring the finely-wrought marble, as ice is shivered by a mattock."

In this passage a profound change from the attitude of the Ecclesiological Society may be noted. No longer is it necessary to present perfection of form as a living testimony to the Christian witness, its continued life is asserted despite the damage inflicted by the wreckers of the past.

Nevertheless, restorations of earlier type continued, notably under the direction of Lord Grim- thorpe (formerly Sir Edmund Beckett Denison, QC). In 1877 matters came to a head over Tewkesbury Abbey, where the aged Scott, despite the sensitivity he could bring to mediaeval masonry, proposed to remove the seventeenth-century furnishings. To a younger generation, they too were part of the


building's history, and over this issue William Morris founded the Society for the Protection of Ancient Buildings and published its Manifesto. This important document, which still forms the basis of the Society's policy, represents a milestone in thinking about the repair of historic buildings, and has dominated attitudes in England for over a century. It has also had considerable influence abroad. In it, Morris proscribed the restoration of a building to an earlier stage of its development, as had been done at the Holy Sepulchre and vast numbers of other mediaeval churches, and emphasized the need for careful and consistent maintenance, or 'daily care'. He stressed too the need to preserve the patina of age. However, unlike Ruskin, whose total opposition to tampering with historic fabric had led him to prefer demolition to repair,23 Morris, in the Man- ifesto, implicitly acknowledged the need for renewal of decayed stone; but, he insisted, where this was done the ne.w should be clearly distinguishable from the old.

The working out of this dictum in practice has been a matter of concern to many architects. In fact, the idea was not completely new, and in restoring the arch of Titus in the Forum Romanum in the early nineteenth century, the architect Giuseppe Valadier had distinguished his new architectural mouldings by omitting the enrichments, and by simplifying the new Ionic capitals whilst retaining the overall form of the antique survivors.24 A similar approach was adopted about 1880 in the reconstruction of the shrine of St Frideswide in Christ Church Cathedral, Oxford (Figure l.6), where the piers supporting the fragments of the canopy are uncompromisingly cut to show their status as new elements. Other examples of this may be seen on the exterior of many buildings, most notably where sculptured corbel- tables or label stops have been renewed but left uncarved as simple projecting blocks of stone. For plain areas of masonry, on the other hand, a convention arose of replacement, not in stone, but with tile, in order to differentiate the repair from

Figure 1.6 Christ Church Cathedral, Oxford, the shrine of St Frideswide as reconstructed about 1880 (F.H. Crossley/ Courtauld Institute)

8 An introduction to the restoration, conswvatio In and repair of stone

Figure 1.7 Buttress repaired by the tile method, St Mary's Church, Higham, Kent

surviving mediaeval work. An example may be seen in the detail illustrated of a buttress at St Mary's Church, Higham, Kent (Figure 1.7), and the technique became widely used in the early years of the present century. Brick had often been employed for the same purpose but for different reasons in the eighteenth century, and it was no doubt felt that the use of tile would result in a similarly pleasing patchwork. Soon, however, it became clear that the character of large areas of mediaeval masonry was being transformed in a way just as assertive as insensitively inserted new stone; and an alternative was therefore developed in which the tile was recessed half an inch from the wall face, and its surface rendered to provide a closer colour match to the adjoining old masonry. The revised method was strongly advocated by A.R. Powys, who, in his book Repair of Ancient Buildings (1929), from which Figures 1.8 to I . I I are taken, linked it especially with the work of William ~ i e r . ~ ~ The first two illustrations show repairs carried out in this way to a buttress, and the second pair the same technique employed in the repair of a mediaeval window at Limpsfield Church, Surrey, in 1927.

Closely related to this method is the use of 'plastic stone', a specially constituted mortar reinforced and

keyed back to sound stone with copper wire or dowels used to make up defective areas. This too, was widely used between the wars, but has problems of its own, and shares with the rendered tile method a tendency to discoloration over a comparatively short period of years, so that even if there is a good match with the old stone when first completed and dried out, it will often weather to produce a disfiguring piebald appearance. The material also requires great care in mixing to avoid being too strong and impervious, and whilst in skilled hands it can be a useful solution in some circumstances, it has acquired a poor reputation amongst many architects since where the mix is too strong it will eventually crack away from the backing stone owing to the effects of salt action. When this happens (Figure 1.12) it will often pull away a further inch or more of old stone with it. For success, the mixture must thus be quite weak; and the material cannot, therefore, be used for the repair of structural or weathering elements. Indeed, it has always been best used for the simple filling of cavities, rather on the principle adopted by the dental profession. In this connection it is interesting to see that Powys seems to have acknowledged that the rendered tile method too was unsuitable for weathering elements, since in his drawing showing the repair of a buttress (Figure 1.8) he shows a new stone for one of the water- tablings. Undoubtedly, however, the greatest danger in the use of plastic stone is the tendency for every blemish, however tiny, in a wall to be 'repaired'; and examples may be found where masonry has been so over-treated with the material that it has lost the patina of age, and thus presents an unpleasant smoothness to the observer.

Attitudes towards the repair of stonework today

From the chronological resume above it will be seen that the repair of masonry has a history of its own, and is not simply a mechanical operation which can be tackled in a purely utilitarian way. Rather, the architect must educate himself to understand the art- historical and archaeological importance and character of the particular structure with which he is concerned, and develop a sensitivity towards the preoccupations of the different groups and discipli- nes interested in its continued preservation. Inevit- ably too, he will come to see his own operation not only in the historical perspective of the individual building, but also in the context of the philosophical developments of the last two or three hundred years.

Perhaps the greatest lesson to be learned from the observation of work carried out during the eighteenth and nineteenth centuries is that the least

Attitudes towards the repair of stonework today 9

NEK

Figures 1.8 and 1.9 Rendering of tiles; diagrams showing of Ancient Buildings, figures 1 1 and 12. Reproduced by stages in the repair of a buttress. (From A.R. Powys, Repair kind permission of Mrs Eleanor Walton)

possible amount of stone renewal, whatever material is selected to replace it, makes for the fewest problems and greatest preservation of historical material. The crucial decision to be made is what to repair and what to leave alone, a critical matter which must be approached in a careful and organized way and not simply left to be made at the last moment by someone on site. Just as an elegant mathematical proof will do only what is necessary and sufficient, so too the historic-buildings architect should adopt the same criteria with respect to aged masonry. The first stage, as emphasized in the first section of this chapter, is a painstaking survey leading to historical appreciation of the structure and careful and thorough diagnosis of any defects which are found. These, as stressed, have to be set into the context of the building as a whole, in dimensions of

both space and time. Obviously too, any major structural movements which are progressive must be attended to at an early stage; but even here careful thought is necessary, and where cracks are the result of movement which took place early in a building's history and have since remained static there may be no need for action, obviating any need to disturb the archaeological integrity of the wall or its foundations. Where work is necessary, on the other hand, it is important it should be carried out using a method which will involve the least damage to historic fabric, not necessarily by the cheapest available. Thus, the use of bored rather than driven piles may avoid damage through vibration; or in instances where facework has become detached from the main mass of a wall and is bulging, a method of tying it back in situ (with, for example, resin anchor bolts concealed


Figures 1.10 and 1.11 Limpsfield Church, Surrey. A and 15. Reproduced by kind permission of Mrs Eleanor window in the north wall during and after repair in 1927. Walton) (From A.R. Powys, Repair of Ancient Buildings, figures 14

in the joints) will often be preferable to taking the face down and rebuilding it. Again, it is often possible to avoid rebuilding dangerously leaning walls by jacking them back into a vertical position or by stabilizing them with concealed reinforced concrete members. For examples where this has been successfully done, the reader is referred to Old Churches and Modem Craftsmanship by A.D.R. Caroe (1949), The Care of Old Buildings by Donald W . Insall (1958), and the same author's The Care of Old Buildings Today (1972).

Once the problems presented by such major structural faults have been resolved, the wall may be looked at in terms of its individual components. The architect should consider each defective stone in

Figure 1.12 Plaxtol Church, Kent. A failed cement repair turn, asking if it is doing its job in the wall as a load- to the belfry window bearing or weathering member. Just because the face


of a stone is weathered it does not mean it has become incapable of supporting the masonry above: after all, it may have lost half an inch of its face in the course of one or more centuries, but there may well 'be considerable substance remaining which could be allowed to weather further for a material period before renewal becomes necessary. Even where a stone is fractured, this does not mean of itself that it is no longer fulfilling its function and must be cut out and replaced. A cracked lintel in the Temple of Zeus at Athens has been cited as an instance of this: the fractured block of marble now acts as an arch rather than a beam and thus remains structurally completely ~table.~%ther fractures are often caused by the rusting of buried iron dowels or cramps which can be carefully removed and the disrupted stone repaired by gluing the broken pieces together using a suitable masonry adhesive.

The special techniques now available for the consolidation of decaying architectural sculpture are discussed in Volume 2. but these are not always

Figure 1.13 Section through a cornice, St Paul's Cathedral, London, showing the method of renewing the decayed corona. (Illustrated by kind permission of Robert Potter) (a) Original stonework cut back; (b) new stone; (c) stainless steel dog cramp across each joint; (d) stainless steel fishtail cramp and dowell; (e) lead weathering

applicable to ordinary building elements. Neverthe- less, a conservative approach may still be adopted for structural members even when they have failed. Thus, it may be possible to flash the pitted upper surface of an eighteenth-century cornice with lead in order to restore its weathering capabilities, or use the same method to provide a drip at its leading edge when that on the soffit of the corona has decayed. Other defective stones may be carefully pieced in to avoid the need to renew the whole, although the situations in which this can be done successfully must be chosen with care in order not to introduce a distracting pattern of fresh joint-lines into the masonry. A good example of the technique in practice is the work recently completed on the western towers of St. Paul's Cathedral, London. Here (Figure 1.13) the corona had weathered, but the mouldings of the cornice which it had sheltered were in good condition. Rather than renew the whole cornice, it was therefore decided to renew only its upper half, making a new joint in the angle beneath the corona where it was concealed in shadow. A similar instance may be found in the 'half- and-half technique for the repair of window mul- lions and tracery which have weathered where exposed to the elements. In such instances, it is often possible simply to cut back the decayed stone to the glazing line, and dowel or glue back a new outer face to the old inner half. This technique was used in St. Anselm's Chapel at Canterbury Cathedral at some date prior to 1845,~' and has been used recently with great success in the Lady Chapel and western tower at Ely (Figure 1.14). Here too, the new joint is concealed, this time by the glazing groove.

This careful approach, seeking always to retain every old element that can possibly continue to do its job, is the hallmark of the competent historic- buildings architect, and contrasts strongly with the attitude commonly displayed by the inexperienced.

Figure 1.14 The half-and-half technique used for repair of a mullion, west tower, Ely Cathedral. (Architects: Donovan Purcell and Peter Miller)

12 An introduction to the restoration, conservatio n and repair of stone

Figure 1.15 Decayed but sound masonry, south porch of St Michael's Church, East Peckham, Kent

For example, some years ago, a restoration of the fine fifteenth-century doorway of the south porch of St Michael's Church, East Peckham, Kent (Figure 1 . 1 5 ) was proposed. Looking critically at the masonry, however, and asking whether each stone was still capable of doing its job showed not only that the arch, although weathered, was structurally sound, but also that the drip provided by its label mould was in a fully functioning state. In this instance there was therefore no need for any stone repair, with a consequent saving not only to parish finances but, most importantly, to the historical integrity of the masonry and its wonderful state of picturesque decay. At St John's Church, Wateringbury, Kent, on the other hand, the drip mould of the west window of the south aisle had decayed to a point where renewal was necessary for the preservation of the window masonry beneath; but at the same time it was clear replacement could be restricted to parts of this element only (Figure 1 . 1 6 ) . Furthermore, no attempt was made to repair the weathered tracery beneath, beyond careful pointing of its open joints and a few unimportant fractures.

The decision to renew any stone must in every case be taken on an individual basis, and only after a close inspection has been made. One should never think in terms of areas for renewal. Final decisions about stones at high levels can, accordingly, only be

Figure 1.16 Repaired drip mould, west window of south aisle, St John's Church, Wateringbury, Kent. (Architects: Purcell Miller Tritton and Partners).

taken once the necessary scaffolding has been erected, and the repair specification must be written with this in mind. There is no place in historic- buildings work for the architect or other professional who remains on the ground, since defects will very often appear in a totally different light once close access is possible, and decisions over renewal should never be delegated. It has also been stressed above that the need for renewal must be assessed in the context of the building as a whole, seen both in its own timescale and that of the progress of its weathering, that is, its overall rate of decay. It must constantly be kept in mind that the purpose of repair is to hand down to the next generation the maximum possible quantity of historic fabric, not to put all defects or potential defects in the masonry to rights and obtain a textbook example of sound construction. Frequency of access, however, plays an important part in decisions over what is 'necessary and sufficient' at any given time, and when the expense of scaffolding is involved it is common to plan for an anticipated period of, say, fifty years before further repairs are needed; whilst for masonry at lower levels it is easy to go back and do a little more in ten or twenty years should this become necessary. The condition of stones on a tall spire or high parapet may, therefore, be rather more critically assessed than those on parts of the building to which more frequent access is possible.

Looking at a building and its decay in both space and time also means that there will be a different assessment of need for renewal of individual stones in a ruin, such as the east wall of Tynemouth Priory, Northumberland (Figure I . 1 7 ) , where erosion is part of the character of the building, and in an eighteenth- century ashlar facade in good condition. As sug- gested above, the ruin may be considered to reflect Burke's character of the Sublime; whilst in the case of the ashlar facade, its smoothness, corresponding with his notion of beauty, is paramount. Both are

ritudes towards the repair of stonework today 13

Figure 1.17 Eroded masonry as part of a ruined character. Tynemouth Priory, Northumberland

characteristics whose retention should be sought, and whilst there would be little point in renewing any of the drip mouldings at Tynemouth in an attempt to slow down the overall rate of decay, there would be a very good case for renewing a defective stone in an ashlar faqade if this would help to prevent the imminent decay of the stone below. Similarly, a nineteenth-century church or extension to an eighteenth-century house will have a character of its own which will call for sensitivity in its preservation. The architect must train himself to respond to this by constant visiting and observation (perhaps aided by drawing or photography) of a wide range of historic buildings.

Altogether, the most important judgement an architect must bring to bear is his assessment of the rate of decay of an individual element. This requires experience, often gained by the quinquennial Survey and resurvey of churches under the Inspection of Churches Measure 1955, and there is no quickly available substitute, although the comparison of old photographs with the state of the structure today can often be helpful. Often, slow decay may be left to take its course for a few more years before repair

becomes necessary, and it is the ability to differentiate this from rapid decay which is important. The expertise required for historic-buildings work is thus very different from that needed in ordinary architectural practice. Much damage has been done to historic fabric by inexpert misjudgement and unnecessarily panicky action. The inexperienced individual, faced with the repair of a masonry structure for the first time, should have no qualms about obtaining a second opinion from an experi- enced architect.

When a decision has been made to replace a particular stone, it will either be cut out completely or to a certain depth. Once the destruction of historic fabric which this involves has taken place, it seems, perhaps, a little academic to consider what is selected to go in its place. Nevertheless, a great deal of discussion over this matter has taken place in the past; and a case can be made out for each of the methods reviewed. Today the choice rests generally between natural and 'plastic' stone, since the use of tiles is generally out of fashion although renewed interest has been shown by one or two individuals over the last few years. Some of the practical considerations affecting the choice between the genuine and the artificial product have been outlined above, the most important undoubtedly being that 'plastic' stone cannot be used for structural or weathering purposes, and can only be used to fill cavities. In many instances, therefore, it may be appropriate to employ both new and 'plastic' stone on the same job, using the former where necessary for structural reasons, and the latter to enable the minimum of old stone to be cut away where a little 'dentistry' will suffice.

Much of the nineteenth- and twentieth-century opposition to the use of new stone has come not merely from over-renewal, but also from its often hard appearance in a weathered wall. Mid- nineteenth-century masonry in particular often exhibits this insensitive character, and it is important that the architect should learn, by observation of old work, to specify replacements correctly. Mediaeval stonework was often comparatively crudely set out, especially in curved work, and the contrast between work of this nature and that of the nineteenth century may be seen clearly in the arcade running round above the wall benches in the Romanesque chapter house and vestibule at Bristol Cathedral (Figure 1.18). Early stonework was, moreover, dressed by hand from the rough block, and it is virtually impossible to obtain the same effect by taking a modern piece sawn die square and tooling or 'distressing' its surface and arrises (Figure 1.19). Of particular importance is the bed joint, which in mediaeval times was often only very roughly dressed, so that as the face of the stone weathers gently back an irregular, undulating joint line is constantly


1.20). It is vital that the architect learn to recognize that such irregularities have nothing to do with weathering, and are a product of the original craftsmanship; when looking at a wall with a view to its repair, the two must constantly be distinguished. The stones illustrated in Figure 1.21 for example, were laid in 1410, and still bear their original tooling, although it is slightly eroded by time. In no way would a modern piece of the same stone weather to the same appearance in five or six centuries if finished with a sawn or rubbed face, and it is not good enough, therefore, to slap in a new stone and hope that it will weather to match the old. Careful attention must be paid to the specification of finishes

Figure 1.18 Irregular setting out of Romanesque work (right) compared with the nineteenth-century renewal on both face and joints so that the character of the

(left). Chapter House, Bristol Cathedral old masonry is matched and the new blends happily and unobtrusively with the old even before weather-

Figure 1.19 The unsatisfactory appearance of distressed masonry, Jumieges Abbey, Normandy

exposed rather than the hard, mathematically precise network which would result from flat, truly-sawn beds and perpends. The point may be appreciated instantly in connection with paving if the old slabs in a London street, with their hand-dressed edges, are compared with new sawn replacements (Figure

ing has commenced. Even after great care has been taken with specifica-

tion, it is essential that the detailed attention which must be given to individual stones is borne upon the contractor and carried through into execution. The way the masonry trade is organized today, however, makes this extremely difficult to achieve since stone is seldom dressed on site. Instead, one man will visit the building to take measurements and profiles which are transferred to cards showing the banker mason the shape and dimensions of the new stone required. The latter will then dress the stone in a shop which may be miles from the building under repair, without ever having visited the site himself, and is thus completely unable to gain any feeling for the character of the masonry and its individual needs. The stone will be fixed by a third man, who all too often is not a mason, and may simply regard the stone as an inconveniently large form of brick. This is a most unsatisfactory state of affairs, but one not easily resolved in many instances. In the case of a large and important historic building, however, a directly employed team of craftsmen may be available, whilst for smaller jobs it may be possible to employ a small local firm who can bring the necessary individual attention to bear.

Great care is also necessary in the specification and execution of pointing, not just from the point of view of mortar mixes, but also on account of the impact the style of pointing will have on the final appearance of the work. Where one or two individual stones are to be replaced it should not necessarily be assumed that the entire wall needs to be repointed, and just as conservative an approach should be brought to bear here as in the renewal of stone. After all, the mortar is as much a part of the archaeology of the building as the building block. In such cases, the aim should be to match the original pointing as closely as possible, and experiments and sample panels for approval should be allowed for in the specification. The same is true where a whole


Figure 1.20 (a) Hand-dressed edges of old York stone paving, City of London; (b) sawn edges of new York stone paving

wall has to be repointed, but here, if the existing pointing is at variance with the character of the building, there is an opportunity to change to another style. This is most commonly done where a building was furnished with 'ribbon' or 'snail-creep' pointing in the nineteenth century, and a change to a more seemly variety is desired. For many mediaeval walls this replacement may be of the 'Ancient Monuments' type, where the mortar is kept back

slightly from the faces of the stones to allow their arrises to read fully; but this is not always appropriate, and the architect should familiarize himseLf with the whole range of alternatives, both modern and historical. Above all, he should be aware of the damage that can be done to masonry in the removal of old pointing, especially where Portland cement has been used in it, and avoid the use of terms such as 'hack out' in his specification. Where possible, the


Figure 1.21 Weathered tooling of 1410, St Bartholomew's Church, Tong, Shropshire

term 'rake out' should be employed; but where forced by the hardness of the pointing to use the expression 'cut out' the architect should stipulate that this is to be done with a hammer and chisel used along the direction of the joint and not across it, in order to avoid damage to the arrises.

Finally, a word must be said about the desirability of preparing record drawings of any wall of archaeological importance both before and after repair. Record drawings may, in any case, be a vital prelude in approaching repair, especially where

renewals and alterations have been carried out in the past, and will enable a proper understanding by the architect of the historic masonry under consideration. It may, indeed, lead him to stay his hand in certain particularly sensitive areas; or, conversely, indicate where repair can be carried out with impunity. It is only recently, for example, that a proper study has been made of the city walls at Canterbury, Kent. Comparison of the photograph (Figure 1.22) with the record drawing (Figure 1.23) will show how features overlooked for centuries have been revealed. Besides the plotting of the individual stones making up the wall, their types were also determined and examination was extended to the mortars used in the different phases of the wall's construction. The most notable discov- ery has been the row of early battlements which are believed'to be of Roman date, repaired in the early Norman period, and later built into the nave wall of the twelth-century church of St Mary Northgate. Many more items of interest have been revealed by careful studies of this nature.28

The post-repair drawing, on the other hand, will be of the very greatest value to those who follow f@ years or a century later, enabling historians to distinguish replacements from old stone with great ease and providing valuable information to whom- soever has the job of supervising the next round of repairs. It may also, incidentally, be useful when

Figure 1.22 The north wall of the twelfth-century nave of St Mary's Church, Northgate, Canterbury, Kent, built on the earlier city wall (Kentish Gazette)


a Roman face intact [TII] C12th heightening C12th window

Late C14th refacing 0 C18 & 19th brick & f l int infilling 0 Obscured by modern rendering

m O 1 5 10 15 20

f t O 3 15 30 45 60 - -,,- ,-- . . .I

Figure 1.23 Interpretation of the wall shown in Figure 1.22 (Canterbury Archaeological Trust)

Figure 1.24 Head of the tower, north face, St Margaret's 'C' marks positions of concealed stainless steel cramps Church, Horsmonden, Kent. Areas dismantled and rebuilt (By kind permission of Purcell Miller Tritton and Partners) are shown dotted; new stones are hatched;

18 An introduction to the restoration, conservatio n and repair of stone

explaining to a client where his money has been spent, since in a good repair the renewals should blend into the wall and be difficult to see. Although in mkny ways the greatest compliment that can be paid to an architect is that the building after repair looks no different than before work commenced, he may still be concerned that there is little to show for what may have been considerable expenditure. Record drawings of this nature need not be particularly elaborate or costly to produce. Those made when the parapets and facings of the upper part of the tower of St Margaret's Church, Horsmonden, Kent, was dismantled and rebuilt in 1971 (Figure 1.24), for example, were prepared as a matter of routine during the course of the contract. They show not only the areas of masonry taken down, but also the locations of the new stainless steel ties and cramps inserted and the very small number of new stones which had to be used.

Altogether, it must be stressed, historic buildings are important artifacts, which provide a wealth of data about past habits, manners, techniques, and aspirations. In some cases they may be the only documents left by a defunct civilization. It is essential, therefore, that the architect entrusted with their repair neither seeks to leave his mark upon them nor forces them to conform with modern standards and practices, but labours instead to preserve, without distortion, the full range of evidence and enjoyment they can provide. An ill- considered refacing of the stretch of the Canterbury City wall illustrated in Figure 1.22, for example, would have completely destroyed the archaeological evidence it contained, leaving it bereft of its historical interest and the life given it by antiquity. In the same way, poorly-matched renewals can be equally destructive of the homogeneity of an eighteenth- century wall (Figure 1.25). The repair of masonry

Figure 1.25 Poor piecing in an eighteenth-century ashlar wall

should accordingly never be undertaken in a wanton manner, but must be a carefully considered process aimed at the preservation of both archaeological data and the aesthetic qualities of the building, including those owing to its age.

References

1. Smith, C.H., 'Lithology; or, Observations on Stone used for Building', Transactions of the Royal Institute of British Architects, 1, Pt. 2, 1842, 129

2. The Architecture of Leon Battista Alberti (tr. James Leoni), 2, f.99v, 1726

3. John Evelyn, A Parallel of the Antient Architecture with the Modern, 2nd edn., 1707, 'An Account of Architects and Architecture', p. 10

4. Op.cit, 1728, p.23 5. Edmund Burke, A Philosophical Enquiry into the

Origin of our Ideas of the Sublime and the Beautiful, 1757

6. Price, Uvedale, An Essay on the Picturesque, new edn, 1796, pp. 62-63

7. Op.cit, p. 24 8. Op. cit, pp.11-13 9. Pugin, A.W.N.,Contrasts, 2nd edn, 1841, p. 74

10. Vide Eastlake, C.L., A History of the Gothic Revival, 1872, p. 120 sqq.

11. Report of the Cambridge Camden Society for MDCCCXL, Cambridge, 1840, pp. 9, 15

12. Report of the Cambridge Camden Society for MDCCCXLI, Cambridge, 1841, p. 40

13. Ibid, p. 39 14. Report of the Cambridge Camden Society for

MDCCCXLII, Cambridge, 1842, p. 23 15. The Ecclesiologist, 1, 1841-2, pp. 5, 29, 143 16. Op.cit, pp. 179, 180 17. Ibid, p. 144 sqq. 18. Letter in Winchester College archives from Butterfield

to Sir William Heathcote dated 1st or 17th June 1861 (quoted from Paul Thompson, William Butterfield, 1971, p. 416)

19. The Ecclesiologist, No. CLXXV, August 1866. p. 2 12

20. Prior, E.S., A History of Gothic Art in England, 1900, p. 289

2 1. Scott, Sir Gilbert, Personal and Professional Recollec- tions (ed. G. Gilbert Scott), 1879, p. 325

22. Scott, George Gilbert, An Essay on the History of English Church Architecture, 1881, p. 147b

23. Ruskin, John, The Seven Lamps of Architecture, 1849, p. 180

24. Vide Linstrum, Derek, 'Giuseppe Valadier et 1'Arc de Titus'. Monumentum 25, No. 1 , 43-71, March 1982

25. Op. cit , p. 77 26. Heyman, Jaques, 'The Gothic Structure', Interdisciplin-

ary Science Reviews, 2 , No. 1, 151-164, March 1977, fig. 23 (p. 163)

27. Willis, Robert, The ArchitecturalHistory of Canterbury Cathedral, 1845, p. 116

28. Tatton-Brown, Tim, 'Canterbury', Current Archaeol- ogy, 6, No. 3, 78-83, June 1978

The nature of building and decorative stones

Francis G. Dimes

Introduction

Stone, the primary building material taken from the crust of the Earth, has been used since the earliest times for convenience, endurance and visual impact. Its use began when man gave up the nomadic lifestyle of a hunter-gatherer and began to build permanent setttlements.

Much of the history of the world's civilizations is recorded in stone. In many instances it is almost the only remaining tangible evidence of a past occupa- tion. The monuments include for instance, the great four-mile long, grey granite menhir avenues of Carnac, the gneiss monoliths of Callanish, the sandstone trilithons of Stonehenge and the volcanic tuff and scoria colossi of Easter Island.

The Egyptians were the earliest people to use stone in large quantities for building.' The pyramids are estimated to contain more than two million blocks of limestone each weighing approximately 2.5 tonnes. Stone was considered to be so important that at one time all the quarries were in royal ownership. Granite, limestone, dolerite, quartzite, schist and breccia are some of the stones used to construct the tombs, temples and palaces along the valley of the Nile.

From the early masonry achievements in Egypt, commencing before 3000 BC and extending over three millennia, an impressive catalogue can easily be assembled which demonstrates clearly the importance of stone to man in his building endeavours. There are the alabaster and limestone reliefs and sculptures of Assyria; the alabaster blocks of the Minoan palaces; the marble and limestone architecture of Greece and Rome. Many stones were

Figure 2.1 The coarsely foliated nature of gneiss is well displayed in the monoliths of Callanish, Isle of Lewis, Scotland. (Photo: John Ashurst)

19

20 The nature of building and decorative stones

Figure 2.2 The Parthenon, on the Acropolis, Athens, Greece. Parian marble, from the Isle of Paros was used for the roof (courtesy of R.H. Roberts)

Figure 2.3 The sculptured wall by the Terrace of the Leper King, Ankor Wat, Cambodia (now Kampuchea) is built of sandstone and of laterite (courtesy of E.A. Jobbins)

exploited during the expansion of the Roman Empire and the building of the great frontier wall of China. There is a profusion of stone buildings and trachyte sculpture left by the Mayan culture as well as the incomparable close-fitting masonry of Cuzco, the Inca capital of Peru. The Angkor Wat (Figure 2.3) and other vast laterite and sandstone buildings of the Khmers of Cambodia are covered with narrative reliefs. Nearly 15 000 tons of drystone walling carved with chevron patterns form the palace site on the granite hill known as Zimbabwe. The cathedrals and fortifications of medieval Europe, Russia and Scandi- navia and of Saracenic Syria, North Africa, Turkey and India involve almost every building and decorative stone known. Stone was the material for the great houses and palaces of the Renaissance and the Classical and Gothic revivals which followed them. These styles were often used for public buildings, industrial buildings and churches in North America, Australia and South Africa as well as in Europe. Even modern buildings are frequently clad in thin stone facings in a way which the Assyrians, Romans or Moguls would have understood.

The repair, maintenance and preservation of this vast heritage of stone is an enormous and sometimes costly business. There are also major problems associated with decay and weathering with which this book is largely concerned. However, the principal characteristics of stone emerging from a study of masonry buildings are those of durability, versatility and of beauty. When man built, and indeed builds, for permanence and for impact, stone is the material chosen.

Definitions

Dimension stone is the term used for a rock that can be quarried, cut and worked to a specified size or shape for use in a building as a structural unit or for use purely as decoration. In this sense the term rock is defined in the dictionary as the solid part of the Earth's crust and the term stone is defined as any piece of rock which has been detached from the Earth's crust. All rocks are aggregates of minerals. Thus such materials as clay, coal and sand are recognized geologically as rocks.

Minerals, in strict scientific definition, are natural inorganic substances with symmetrical crystal forms which reflect internal atomic structures and which have defined chemical compositions. Over 2500 minerals have been identified and named. Many of them are rare. Only about twenty-five, either singly or in association, make up the physical bulk of most rocks used for building.'

Distribution 2 1

Criteria for use

Three criteria may be considered to determine whether stone should be used in any particular situation. Firstly, it should be sufficiently durable for the intended purpose. In the past the durability of a stone was discovered from experience of its use, a method of assessment which should not be forgotten today. Now physical and chemical tests can provide valuable additional indications of likely durability (see Chapter 10). Secondly, it should be economi- cally available and easily quarried and worked to the desired profiles. This criterion is still important but is less critical now than in the past because of improvements in transport systems and increasing sophistication of cutting equipment. Thirdly, it should be pleasing to the eye. Because stone is a natural material all types may claim to satisfy this criterion, although it may be noted that some man- made juxtapositions of stone are not aesthetically pleasing.

It may usually be assumed that in the past availability was of paramount importance. Other historical factors relating to political boundaries, ownerships, trade agreements and conditions of instability and war have obviously influenced the use and choice of particular stones.

Geological factors

Geological factors now decide whether a stone may be used within the determined criteria. Geology is the fundamental science which determines not only the scenery of any region but also its architecture. It is the science of the prime natural materials used for building and is the determining factor of regional forms of building. It is immediately apparent that the shape and size of flint, whether used as 'field flint', 'cobble' or 'squared flint', largely dictates the method of construction and is a major influence on the style of building. The main factor limiting the construction method and determining the appearance is the size of the flint blocks that can be obtained and used. The size of these blocks is a direct reflection of the mode of formation, or the genesis, of flint. Similarly, other building stones can only be obtained in sizes which are a reflection of their geological history. It follows, therefore, that any stone chosen for building must be obtainable in blocks large enough for the desired purpose. It should be free from fractures. It should be sufficiently tough and free of minerals which may break down chemically or by weathering. Hardness is not necessarily a requisite, although when a stone is to be used for paving or steps resistance to abrasion is a desirable quality. The distinction

between toughness and hardness should be noted. A tough stone is not necessarily hard.

In some instances, for example where stone is to be used for internal decorative facing, it should be capable of taking a polish. The colour of the stone and its 'figuring' then become important characteristics. Depending upon the architectural detailing of a building, low water absorption, and a macroporous structure may be desirable qualities. It should be noted particularly that no two blocks of stone, even if quarried side by side, are absolutely identical any more than, for example, two planks of oak are. The differences may not be discernible and may be of no practical importance; but they may be substantial. The differences may also contribute greatly to the attractiveness and beauty of the stone and can be exploited to show the material to its best advantage.

Distribution

A purely superficial glance at the geological map of Great ~ r i t a i n ~ shows that the country has a great variety of rocks. This is a reflection of the geological history of the country. All rocks have been used in one manner or another for building purposes. The vast variety of rock types and the sometimes restricted area in which some occur and were used precludes mention of them all; a meaningless catalogue would result. Discussion has been confined, therefore, to those stones which have been used on any scale for building and to those which have

Figure 2.4 In an area without a supply of rock suitable for building, any stones available may be used. The 'conglomerate wall' enclosing the Nursery, Battersea Park, London, in addition to bricks and tiles has been built of many pieces of different types of stone, mostly gathered from around London's docks


particular qualities worthy of note. Many of the stones discussed here are from British sources. But, because Britain is made up of rocks which belong to the majority of all the known types, the geological considerations discussed may be applied world-wide.

On the Geological Map of the United Kingdom, published by the Ordnance Survey for the British Geological Survey, a sedimentary formation is shown as one colour throughout the length of its outcrop. This must not be interpreted as indicating that the type of rock is consistent throughout that outcrop. This is because the map shows the age of the formation, not necessarily its lithology. The type of stone in any area cannot be identified by reference to the map alone.

Classification

Despite the apparently bewildering variety of rocks, any one can be placed into one of three groups; all rocks within any one group have common characteristics which are unique to that group. The groups are igneous, sedimentary and metamorphic rocks.

Igneous rocks

The rocks within the lithosphere (the solid outer shell of the Earth, which includes the crust) are normally solid. They melt only when there is a

Table 2.1 Classification of igneous rocks

decrease of pressure or there is an addition of other .material. The molten rock material then formed is termed magma and it may originate at different levels within the Earth. It is essentially a fluid silicate melt with water-vapour and other volatiles. In geologically favourable conditions, magma may rise through the crust, becoming lighter through expansion and increasingly mobile. On cooling, at whatever level in the Earth, it forms an igneous rock. Some of the magma may have poured out on the surface as lava during a volcanic eruption and the resultant rock is known as volcanic or extrusive igneous rock. Magma which cools and consolidates within the Earth is termed an intrusive or plutonic igneous rock and is seen only when the encompas- sing country rock has been weathered away. No matter where they are found, igneous rocks have characteristics directly arising from the cooling and consolidation of magma. The form, or shape, will depend on where the magma came to rest. The rate of cooling also depends, to a great extent, upon the position of the magma within the Earth. If poured out onto the surface the magma will cool rapidly and glassy or very finely crystalline volcanic igneous rocks will result. Many cubic kilometres of magma within the lithosphere may be contained in chambers. It cools slowly and coarsely crystalline plutonic rocks result, the crystals of which normally can be individually distinguished by eye. In some instances, the magma cooled at variable rates and large crystals,

Position of emplacement Chemical composition

Acid Intermediate Basic Ultra- basic

Volcanic (extrusive) (glassy or very fine-grained)

Normally these rocks are glassy or too fine-grained for individual crystals to be seen

G Tuff m

Pumice Andesite Basalt* Obsidian* Rhyolite

Minor intrusions Quartz obviously Some quartz may No quartz No quartz (fine-grained or medium-grained) present be present seen seen

Quartz-porphyry Porphyry* Dolerite?

Plutonic Granodiorite* Diorite* Gabbro* Serpentinite* (medium grained or coarse-grained) Granite* Syenite* Peridotite

'The geological names of rocks which have been used on any scale for building, either structurally or decoratively ?Quartz dolerites do exist

Classification 23

termed phenoclysts were formed first. The remaining magma cooled more slowly and the phenocrysts may be surrounded by smaller crystals.

Igneous rocks, therefore, can be classified using the position of emplacement as the criterion. However, this does not take account of the chemical composition of the original magnla, which can be deduced from the chemical cornposition of the minerals which crystallize from it.

Igneous rocks are essentially assemblages of sili- cates. When they are chemically analysed the proportion of silicon dioxide (SiOz) present may be used as a basis of classification. Those rocks yielding a high percentage of SiO, are termed acid. These rocks contain the mineral quartz, the crystal form of silica. The term acid refers to the chemical composition of the rock and does not imply that the rocks have a corrosive quality.

Rocks which yield a low percentage of SiO, are known as basic and as ultra-basic. The terms basic and ultra-bisic refer only to the chemical composition of the rocks and not to their origin.

Purely arbitrary limits may be set for the percentage of silica present. A common classification is shown below:

Rock type Per cent SiO,

Acid >65 Intermediate 5 5 4 5 Basic 45-55 Ultra-basic


Table 2.2 Classification of sedimentary rocks

Rock type Name Main constituents Remarks

Rudaceous Breccia Large fragments of Broken, angular, mostly unworn fragments (rubbly rocks mainly any rock type set in finer material and held together by composed of large natural cement; often a cemented scree. fragments of older Conglomerate Large fragments of Rounded fragments in finer material and rocks) any rock type held together by natural cement;

a cemented gravel.

Arenaceous Sandstone* Quartz grains

(sandy rocks) Grit (stone)' Quartz grains

Flagstone* Quartz grains and mica flakes

Arkose* Quartz grains and feldspar, commonly partly decomposed

Quartzite* Quartz grains

Bedded, composed of rounded quartz grains, fine to medium grained, usually with grains cemented. Bedded, composed of angular quartz grains, usually medium to coarse grained. May contain small pebbles. Generally coarsely bedded. Finely bedded, fine-grained rounded quartz, with layers of mica flakes lying along bedding planes. Sandstone or grit, medium to coarse grained containing over 25 % feldspar. Mostly terrestrial deposits. Composed almost entirely of quartz grains, closely fitting and naturally cemented with silica.

Argillaceous Clay (clayey rocks)

Mudstone

Shale

Clay mud

Clay mud

Clay mud

Very fine-grained, flaky minerals, structureless. Clay with much water squeezed out; very fine grained, massive and structureless. Laminated, commonly finely compacted mudstone. Splits along laminae which are in the direction of the original bedding.

Calcareous Limestone* (carbonate rocks mainly of calcium and Oolitic magnesium carbonate) limestone* Some calcareous Magnesian rocks are limestone* chemically precipitated. Tufa*

Calcium carbonate, (calcite) Ooliths of calcium carbonate Magnesium carbonate and calcium carbonate Calcium carbonate

Travertine* Calcium carbonate

Bedded, composed essentially of calcium carbonate Limestone composed mostly of small spheroidal calcareous grains. Limestone, with a high proportion of dolomite; massive, granular, saccharoidal. Deposited from saturated h e y waters; M l e , porous (spongy) structure. Similar to tufa, but more compact, more dense, not friable.

Organic Coal Lignite Peat

Organic remains Bedded rocks formed from vegetable matter.

Evaporites Gypstum Hydrated calcium Chemically precipitated from (includes Alabaster*) sulphate evaporating waters Anhydrite Calcium sulphate Rock salt Sodium chloride

Chemical precipitates Chert *f Silicon dioxide with Flint* greater or lesser Jasper amounts of (some limestones are impurities chemical precipitates)

'The geological names of those rocks which have been used on any scale for building, structurally or decoratively tThere is geolo

John Ashurst - Conservation of Building and Decorative Stone

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